1. Field of the Invention
The present invention relates to an apparatus and method for the controlled cooling of chemical tanks.
2. Discussion of the Related Art
In microelectronics industry, the manufacturing of integrated circuits involves several steps of liquid-based or "wet" etching: the silicon wafers are immersed in rather capacious tanks (to allow the simultaneous processing of several wafers) containing a chemical solution whose composition and temperature depend on the particular material to be etched.
In some cases the etching process is carried out at temperatures higher than the room temperature (21.degree. C.): the etching solution must be maintained within a temperature range from 28.degree. C. to 38.degree. C., for which purpose means for heating the etching solution are necessary.
There is a class of materials for which the etching process is carried out at room temperature. In this class, some materials, such as silicon, react with the etching solution generating heat (i.e., the reaction is exothermic). If no cooling means are provided, the heat generated by the reaction causes an increase in the temperature of the etching solution which modifies (increases) the reaction speed, and the etching process goes out of control. As a result, several wafers must be rejected.
Also, as evident, cooling means are necessary when the etching process is carried out at temperatures lower than the room temperature.
According to the known art, the cooling means include teflon coils which are immersed in the etching solution and through which water is made to flow. The coils are connected to an external tank to which a refrigerating system is associated. A temperature sensor immersed in the etching solution controls the refrigerating system to maintain the etching solution in the desired temperature range.
The problem is that, with use and handling due to cleaning, microfractures are created in the teflon coils, through which the water leaks out contaminating the etching solution with ions present in the water. Such ions deposit on the silicon wafers and give rise to defects that cause the wafers to be rejected.
In the published European Patent Application No. 507387 a device for heating chemical tanks is described which includes a pair of heat-exchanger teflon coils, immersed in the etching solution, through which an inert gas such as pure nitrogen is made to flow. The coils are connected to respective heating units which are fed with the inert gas from a main supply line. The use of an inert gas as the heating element instead of water assures that, even in presence of microfractures in the teflon coils through which the gas can leak, the etching solution is not contaminated.
In view of the state of the art described, it is an object of the present invention to provide an apparatus and method for the controlled cooling of chemical tanks used for wet etching of semiconductor wafers, which is safe and reduces the chance of contamination of the etching solution.